MEMS devices generally include a single or composite micro-mechanical structure, which not only have electrical function, but also have mechanical, chemical, optical and biological function. An integrated system including a MEMS device and a semiconductor circuit can achieve a particular function. MEMS devices are widely applied in sensors or controlling systems, or for driving mechanical, optical or chemical movement in micro movement field. MEMS devices can also be used as an independent unit or system. A kind of widely used MEMS devices or systems measure inertia forces by translation, rotation or inertial sensing, such as MEMS acceleration sensors, gyroscopes.
A conventional capacitive MEMS inertial sensor includes a pair of inertial electrodes and a pair of fixed electrodes. A major portion of an inertial electrode hangs over a carrier of the device, and a small portion of an inertial electrode is fixed on the carrier. The fixed electrodes which are opposite to the inertial electrodes are fixed on the carrier and are required to have a great rigidity. There are narrow air gaps formed between the inertial electrodes, especially the portions hanging over the carrier, and the fixed electrodes, which form a pair of capacitors.
When an inertial force is provided in a certain direction, the potions of the inertial electrodes hanging over the carrier moves along the direction of the inertial force, that is, moves relative to the fixed electrodes, which changes the capacitance of the capacitors formed by the inertial electrodes and the fixed electrodes. By measuring the capacitance change, the movement between the inertial electrodes and the fixed electrodes may be obtained, and so does the inertial force. Therefore, capacitive MEMS inertial sensors are widely used in automobile industry, manufacture, consumer products industry and handheld electronic devices, nowadays. The conventional MEMS inertial sensors are mostly formed with bulk silicon MEMS, thick or thin silicon-on-insulator (SOI) manufacturing process, in which one or more suspended inertial electrodes are connected to a carrier, and inertial electrodes and fixed electrodes opposite to the inertial electrodes are isolated with air gaps.
Compared with the inertial electrodes, the fixed electrodes should have a great rigidity, so that under an inertial force, the fixed electrodes remain static or move a very little distance, thereby ensuring the accuracy of calculation the inertial force by measuring the capacitance change caused by the movement of the inertial electrodes. However, due to the limitations of the manufacturing process, it is difficult to fix a whole fixed electrode to the carrier in a conventional MEMS inertial sensor, and only a portion of the fixed electrode can be fixed on the carrier, thus the fixed electrode is still elastic and deformable and may move wholly or partly under the inertial force. Therefore, although having different rigidity and dynamic characteristics, both of the inertial electrode and the fixed electrode will move or vibrate. Then, the change in capacitance between the inertial electrode and the fixed electrode depends on the changes in position of the inertial electrode and the fixed electrode, rather than the distance the inertial electrode moves to the carrier, which may adversely influence the accuracy of the MEMS inertial sensor.
The process for manufacturing the conventional inertial sensor includes: forming a sacrificial layer before forming an inertial electrode and a fixed electrode, selectively removing a portion of the sacrificial layer after forming the inertial layer, so that the inertial electrode is suspended, and removing a portion of the sacrificial layer beneath the inertial electrode and the fixed electrode, so that a portion of the fixed electrode opposite to the inertial electrode is also suspended. Due to limitations of the manufacturing process, the fixed electrode can not be entirely fixed on the carrier, and only a fixing portion of the fixed electrode may be fixed on the carrier, which may cause a deformation relative to the carrier under an inertial force. The deformation may adversely affect the measurement of the capacitance and thus the measurement of the inertial force. Without regard to it, the deformation of the fixed electrode will become a dynamic noise and may cause measuring errors. Therefore, the deformation of the fixed electrode limits the accuracy of the conventional MEMS inertial sensor.
Nowadays, the MEMS inertial sensor are manufactured with an optimized process, such as, integrated with a CMOS readout integrated circuit (ROIC) on a same semiconductor substrate, so that the MEMS inertial sensor is embedded in the CMOS circuits. Compared with the traditional process in which MEMS devices are separately manufactured with ROIC, the optimized process may reduce the size, weight and power consumption of the MEMS devices, and improve the efficiency of the application system.
However, it is difficult to form a capacitive MEMS inertial sensor on a substrate including CMOS RIOC, especially on a thin silicon substrate compatible with the CMOS process.